1) Field of the Invention
The present invention relates to a helical antenna for portable radio communication equipment. More specifically, the present invention relates to a small helical antenna for a portable transmitter/receiver or a pocket telephone (mobile telephone) of a small power type used for an in-plant communication system or a tele-terminal.
2) Description of the Related Art
Recently, according to developments in radio communication equipment, a number of communication systems have adopted a radio communication system instead of using a wired system. As a result, there are no useable frequencies left in the low frequency band, so that gradually higher frequencies are being assigned for new radio communication systems, for example, frequency bands of 400 MHz to 800 MHz are assigned. It is now being planned to use a 1500 MHz band for a relational radio communication system as described above, as explained hereinafter.
In this way, as the frequency used for a radio communication system gets higher, the length of the antenna required gets shorter and the size gets smaller. However, as the size of the antenna gets smaller, it becomes more difficult to obtain a desirable antenna directivity.
Conventionally, a whip antenna that has a small-diameter and a vertical rod, and a helical antenna that has a coil shape and is mounted perpendicular to a flat metal-plate reflector, are used especially in mobile communications, portable radio and television receivers, field-strength meters, and the like. A dimensional relation between the whip antenna or the helical antenna and the casing thereof is different in accordance with the transmitting/receiving frequency required for the antenna. Usually, a casing of radio communication equipment having the whip or helical antenna is not designed in accordance with the optimum radiation therefrom but is designed in accordance with the performance and the output power of the equipment.
Accordingly, in the conventional antenna, as the transmitting/receiving frequency required for the antenna gets higher, the antenna does not provide the desired directivity. Further, in conventional radio communication equipment having an antenna, a return current from the antenna flows in the casing of the radio communication equipment, so the directivity of the antenna changes when the casing is held by a human hand.